Humboldt thus beautifully describes this phenomenon:
The intensity of this light is at times so great, that Lowenörn
(on June 29, 1786) recognised its coruscation in bright sunshine.
Motion renders the phenomenon more visible. Round the point in
the vault of heaven which corresponds to the direction of the
inclination of the needle the beams unite together to form the
so-called corona, the crown of the Northern Light, which encircles
the summit of the heavenly canopy with a milder radiance and
unflickering emanations of light. It is only in rare instances that
a perfect crown or circle is formed; but on its completion, the
phenomenon has invariably reached its maximum, and the radiations
become less frequent, shorter, and more colourless. The crown, and
the luminous arches break up; and the whole vault of heaven becomes
covered with irregularly scattered, broad, faint, almost ashy-gray,
luminous, immovable patches, which in their turn disappear, leaving
nothing but a trace of a dark smoke-like segment on the horizon.
There often remains nothing of the whole spectacle but a white
delicate cloud with feathery edges, or divided at equal distances
into small roundish groups like cirro-cumuli.--_Cosmos_, vol. i.
Among many theories of this phenomenon is that of Lieutenant Hooper,
R.N., who has stated to the British Association that he believes “the
Aurora Borealis to be no more nor less than the moisture in some
shape (whether dew or vapour, liquid or frozen), illuminated by the
heavenly bodies, either directly, or reflecting their rays from the
frozen masses around the Pole, or even from the immediately proximate
snow-clad earth.”